Re: [PATCH v2 0/5] pid: add pidfd_open()

[Jonathan Kowalski]

On Sat, Mar 30, 2019 at 5:52 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> On Sat, Mar 30, 2019 at 05:50:20PM +0000, Jonathan Kowalski wrote:
> > On Sat, Mar 30, 2019 at 5:24 PM Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Sat, Mar 30, 2019 at 10:12 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
> > > >
> > > >
> > > > To clarify, what the Android guys really wanted to be part of the api is
> > > > a way to get race-free access to metadata associated with a given pidfd.
> > > > And the idea was that *if and only if procfs is mounted* you could do:
> > > >
> > > > int pidfd = pidfd_open(1234, 0);
> > > >
> > > > int procfd = open("/proc", O_RDONLY | O_CLOEXEC);
> > > > int procpidfd = ioctl(pidfd, PIDFD_TO_PROCFD, procfd);
> > >
> > > And my claim is that this is three system calls - one of them very
> > > hacky - to just do
> > >
> > >     int pidfd = open("/proc/%d", O_PATH);
> > >
> > > and you're done. It acts as the pidfd _and_ the way to get the
> > > associated status files etc.
> > >
> > > So there is absolutely zero advantage to going through pidfd_open().
> > >
> > > No. No. No.
> > >
> > > So the *only* reason for "pidfd_open()" is if you don't have /proc in
> > > the first place. In which case the whole PIDFD_TO_PROCFD is bogus.
> > >
> > > Yeah, yeah, if you want to avoid going through the pathname
> > > translation, that's one thing, but if that's your aim, then you again
> > > should also just admit that PIDFD_TO_PROCFD is disgusting and wrong,
> > > and you're basically saying "ok, I'm not going to do /proc at all".
> > >
> > > So I'm ok with the whole "simpler, faster, no-proc pidfd", but then it
> > > really has to be *SIMPLER* and *NO PROCFS*.
> > >
> >
> > (Resending because accidently it wasn't a reply-all)
> >
> > If you go with pidfd_open, that should also mean you remove the
> > ability to be able to use /proc/<PID> dir fds in pidfd_send_signal.
> >
> > Otherwise the semantics are hairy: I can only pidfd_open a task
> > reachable from my active namespace, but somehow also be able to open a
>
> You can easily setns() to another pid namespace and get a pidfd there.
> That's how most namespace interactions work right now. We already had
> that discussion.

Only if it is a child namespace, or you have the relevant capabilities to setns.

Currently, if I just put a task in PID namespace, it can see /proc of
an ancestor PID namespace, and opendir /proc/<PID>, this is accepted
by pidfd_send_signal.

If you ever allow signalling across PID namespaces (because file
descriptors should be able to do that, they are not namespaced, see
files, sockets, etc), it will become a problem. Getting pidfds from
outside my active namespace should require userspace cooperation.

So, opening a pidfd should be limited to what *I* can see in my
namespace, like every other namespace. That is what a namespace is,
and PIDs have their own namespace, they're not exposed in the
filesystem namespace.